1. Field of the Invention:
This invention relates to the production of vinyl acetate, and more particularly, to the production of vinyl acetate integrated in a complex process that achieves simultaneous production of vinyl acetate.
2. Description of the Prior Art:
In the wake of "oil crisis", the price of crude oil increased greatly, and as a result, the prices of petrochemical building blocks such as ethylene and propylene soared. In consequence, ethylene and propylene derivatives are losing the competitive power, and people in the industry even see the necessity of manufacturing some derivatives from other sources by processes other than the conventional ones. In the development of technology for producing ethylene and propylene derivatives from sources other than crude oil by unconventional methods, an important role is expected to be played by natural gases which are C.sub.1 compounds per se, and other C.sub.1 compounds such as a synthesis gas (a mixture of carbon monoxide and hydrogen), and carbon dioxide produced by gasification of asphalt and solid fossil fuels such as coal, as well as methanol derived from these gases. Utilization of these C.sub.1 compounds is one of the top-priority objects that must be achieved as soon as possible to meet the demand for less dependency on petroleum and use of more varigated sources of ethylene and propylene derivatives.
Ethanol is known as an industrial chemical and basically it has been manufactured by fermentation of natural products such as molasses or by hydration of ethylene in the presence of an acid catalyst such as phosphoric acid. As already mentioned, ethylene is one of the petrochemicals which are getting more and more expensive. Coal and natural gas are two ample natural resources, and both of them can be converted to a synthesis gas (CO+H.sub.2) from which methanol can be produced. The methanol further reacts with carbon monoxide and hydrogen under appropriate conditions to form ethanol, acetaldehyde, dimethyl acetal and methyl acetate [see, for example, Japanese Patent Publications Nos. 15692/66, 2525/73, Japanese Patent Public Disclosures Nos. 149213/76, 136110/77, 136111/77, 133914/76, Bull, Chem. Soc., Jpn., 52 (2), 479-482 (1979)].
Like ethanol, vinyl acetate is a well known industrial chemical. It is one of the oldest vinyl monomers and polyvinyl acetate and PVA (polyvinyl alcohol) are useful synthetic polymers, so there are many different methods to produce it on an industrial scale. In old days, vinyl acetate was typically synthesized from acetylene, and these days, the acetylene process is being displaced by a more advantageous ethylene process. The first concept of direct synthesis of vinyl acetate from ethylene used a palladium chloride catalyst or sodium acetate in the reaction system. Alternative methods were proposed wherein vinyl acetate and acetic acid were produced from acetaldehyde and acetic anhydride either directly or through ethylidene diacetate (1,1-diacetoxyethane) [see, for example, Hydrocarbon Process, 44 (11) 287 (1965), British Pat. No. 1,112,555, U.S. Pat. Nos. 2,021,698, 2,425,389 and 2,860,159]. In these methods, acetaldehyde and acetic anhydride are reacted directly to form vinyl acetate and acetic acid or they first react to form ethylidene diacetate which is then thermally decomposed to vinyl acetate and acetic acid.
Acetic anhydride used as one of the two starting materials in these methods has been commercially produced by the Wacker process wherein acetone or acetic acid is thermally decomposed to ketene which is then reacted with acetic acid to produce acetic anhydride. Methods have been proposed recently that produce acetic anhydride directly from methyl acetate and carbon monoxide by reacting them in the presence of a carbonylation catalyst and a halide (see, for example, Japanese Patent Publication No. 3926/77, Japanese Patent Public Disclosures Nos. 65709/76 and 59214/79).
Acetaldehyde, the other material for the production of vinyl acetate by thermal decomposition is an industrial chemical that is commercially produced mainly by direct oxidation of ethylene or light hydrocarbons. In other words, like ethanol, vinyl acetate or acetic anhydride, acetaldehyde is also supplied to the market as a product produced from petrochemicals whose prices keeps going up. Methods have therefore been proposed to produce C.sub.2 compounds directly from a synthesis gas (see, for example, Japanese Patent Public Disclosures Nos. 80806/76 and 80807/76). These methods constitute an attractive process for synthesizing C.sub.2 compounds, considering only the starting material. But unfortunately, the process involves a gas-phase reaction using a solid catalyst one component of which is rhodium, so metallic rhodium in the solid catalyst changes to volatile rhodium carbonyl which escapes out of the reaction system until the catalytic activity is lost. This defect is fatal to the process if it is to be performed on an industrial scale, and a considerable time will be necessary to eliminate such defect.
According to more viable methods, C.sub.2 compounds such as acetaldehyde and ethanol are produced from methanol and synthesis gas, as taught in, say, Japanese Patent No. 2525/73, Japanese Patent Public Disclosures Nos. 149213/76, 136110/77, 136111/77, and Bull, Chem. Soc. Jpn., 52 (2), 479-482 (1979). The reactions that occur in these methods for producing ethanol, acetaldehyde, methyl acetate and dimethyl acetal are schematically represented by the following formulae:
(1) CH.sub.3 OH+CO+2H.sub.2 .fwdarw.CH.sub.3 CH.sub.2 OH+H.sub.2 O
(2) CH.sub.3 OH+CO+H.sub.2 .fwdarw.CH.sub.3 CHO+H.sub.2 O
(3) 2CH.sub.3 OH+CO.fwdarw.CH.sub.3 COOCH.sub.3 +H.sub.2 O
(4) 3CH.sub.3 OH+CO+H.sub.2 .fwdarw.CH.sub.3 CH(OCH.sub.3).sub.2 +2H.sub.2 O
As understood from these reaction formulae, the conventional methods for producing ethanol and acetaldehyde by hydrocarbonylation of methanol is disadvantageous in that it produces a great amount of methyl acetate or dimethyl acetal other than the desired products. Attempts have been made to improve the catalyst or increase the yield of ethanol and acetaldehyde by recycling the by-products, but none of them have proved completely satisfactory for practical purposes.